Pyrometry: A Practical Treatise on the Measurement of High Temperatures. Charles R. Darling
directly or indirectly, from the gas thermometer. In the table on next page, a number of fixed points, determined by various observers, is given; the error, even at the highest temperatures, probably not exceeding ±2° C.
In preparing the temperature scale of a pyrometer for practical use, the instrument is subjected successively to a number of the temperatures indicated in the table, and in this manner several fixed points are established on its scale. The space between these points is then suitably subdivided to represent intermediate temperatures.
Table of Fixed Points.
| Substance. | Physical Condition. | Deg. | Deg. |
| Cent. | Fahr. | ||
| Water (ice) | At Melting Point | 0 | 32 |
| Water | ” Boiling ” | 100 | 212 |
| Aniline | ” ” ” | 184 | 363 |
| Naphthalene | ” ” ” | 218 | 424 |
| Tin | ” Melting ” | 232 | 449 |
| Lead | ” ” ” | 327 | 620 |
| Zinc | ” ” ” | 419 | 786 |
| Sulphur | ” Boiling ” | 445 | 833 |
| Antimony | ” Melting ” | 631 | 1167 |
| Aluminium | ” ” ” | 657 | 1214 |
| Common Salt | ” ” ” | 800 | 1472 |
| Silver (in air) | ” ” ” | 955 | 1751 |
| Silver (free from oxygen) | ” ” ” | 962 | 1763 |
| Gold | ” ” ” | 1064 | 1947 |
| Copper (in air) | ” ” ” | 1064 | 1947 |
| Copper (Graphite covered) | ” ” ” | 1084 | 1983 |
| Iron (pure) | ” ” ” | 1520 | 2768 |
| Palladium | ” ” ” | 1549 | 2820 |
| Platinum | ” ” ” | 1755 | 3190 |
It is necessary to point out that the figures given in the table refer only to pure substances, and that relatively small quantities of impurities may give rise to serious errors. The methods by which the physical condition to which the temperatures refer may be realised in practice will be described in the succeeding chapter.
National Physical Laboratory Scale.—Exact agreement with regard to fixed points has not yet been arrived at in different countries, and an effort to co-ordinate the work of the National Physical Laboratory, the United States Bureau of Standards, and the Reichsanstalt, with a view to the formation of an international scale, was interrupted by the war. In 1916 the National Physical Laboratory adopted a set of fixed points on the Centigrade thermodynamic scale, in conformity with which all British pyrometers have since been standardised. It will be seen that the figures differ very slightly from those given in the previous table, which represent the average results of separate determinations in different countries.
National Physical Laboratory Scale (1916)
| Substance. | Physical Condition. | Deg. | Deg. |
| Cent. | Fahr. | ||
| Water (ice) | At Melting Point | 0 | 32 |
| Water | ” Boiling ” (760 mm.) | 100 | 212 |
| Naphthalene | ” ” ” ” | 217·9 | 424 |
| Benzophenone | ” ” ” ” | 305·9 | 582 |
| Zinc | At Melting Point | 419·4 | 787 |
| Antimony | ” ” ” | 630 | 1166 |
| Common Salt | ” ” ” | 801 | 1474 |
| Silver (in reducing atmosphere) | ” ” ” | 961 | 1761 |
| Gold | ” ” ” | 1063 | 1945 |
| Copper (in reducing atmosphere) | ” ” ” | 1083 | 1982 |
For higher temperatures the melting points of nickel (1452° C.) and palladium (1549° C.) are employed, but the accuracy in these cases is not so certain as with the substances named in the table. A useful point, intermediate between copper and nickel, has been established by E. Griffiths, and is obtained by heating nickel with an excess of graphite, when a well-defined eutectic is formed which freezes at 1330° C., or 2426° F.
Temperatures above the Present Limit of the Gas Thermometer.—As it is not yet possible to compare an instrument directly with the gas thermometer above 1550° C., all higher temperatures must be arrived at by a process of extrapolation. By careful observation of a physical change at temperatures up to the limit of 1550° C., the law governing such change may be discovered; and